Batteries are a useful source of stored energy that can be incorporated into a number of systems. Rechargeable lithium-ion (“Li-ion”) batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. In particular, batteries with a form of lithium metal incorporated into the negative electrode afford exceptionally high specific energy (in Wh/kg) and energy density (in Wh/L) compared to batteries with conventional carbonaceous negative electrodes.
A typical Li-ion cell contains a negative electrode, a positive electrode, and an electrolyte or solvent that serves as a separator between the negative and positive electrodes. Typically, during charging, a given amount of electrons are generated at the positive electrode and an equal amount of electrons are consumed at the negative electrode; these electrons are transferred back and forth via an external circuit that connects the two electrodes. During discharging, the exact opposite electrochemical reactions occur.
Batteries with a lithium metal negative electrode afford exceptionally high specific energy (in Wh/kg) and energy density (in Wh/L) compared to batteries with conventional carbonaceous negative electrodes. See, e.g, Armand, M., et al., Building Better Batteries; Nature 451(7179): 652-657 (2008); Flandrois, S., et al, Carbon Materials for Lithium-ion Rechargeable Batteries; Carbon 37(2):165-180 (1999); and Tarascon, J. M. et al., Issues and Challenges Facing Rechargeable Lithium Batteries; Nature 14(6861): 359-367 (2001). Therefore, Li-based batteries are the most promising energy-storage technology for deployment in hybrid electric vehicles (“HEVs”) and electric vehicles (“EV”). Although existing Li-ion batteries may satisfy the need for short-range electric vehicles; long-range (driving range above 300 miles) electric vehicles with a range approaching that of present day vehicles will require systems with increased energy densities. Wagner, F. T., et al., Electrochemistry and the Future of the Automobile, The Jour. of Physical Chemistry Letters, 2010. 1(14): p. 2204-2219.
In one of the earliest papers on the Li-air system, Abraham and Jiang used a Li+ conductive organic polymer electrolyte and a positive electrode with an electronically conductive carbon matrix and containing a catalyst to aid with the reduction and oxidation reactions. Abraham, K. M., et al., A Polymer Electrolyte-based Rechargeable Lithium/oxygen Battery; Jour. of Electrochem. Soc., 143(1):1-5 (1996). Under these conditions, the concept of a nonaqueous rechargeable Li/O2 battery was demonstrated. Over the course of the past ten years many more publications have investigated the Li-air system as a proposition of a high-energy rechargeable system for electric vehicle applications. See, e.g., Beattie, S. et al., High-Capacity Lithium-Air Cathodes, Jour. of Electrochem. Soc., 156: p. A44 (2009); Ogasawara, T., et al., Rechargeable Li2O2 Electrode for Lithium Batteries, Jour. of Electrochem. Soc., 128(4): 1390-1393 (2006); Kumar, B., et al., A Solid-State, Rechargeable, Long Cycle Life Lithium-Air Battery; Jour. of Electrochem. Soc. 157: 50 (2010).
However, at the present, there are still significant challenges that must be addressed for the Li-air system to become commercially viable. For example, Li-air batteries generally suffer degradation mechanisms that limit their useful life-cycle operation. Christensen, J., et al., A Critical Review of Li/Air Batteries, Jour. Electrochem. Soc. 2012, Volume 159, Issue 2, Pages R1-R30. Additional challenges include reducing the hysteresis between the charge and discharge voltages, limiting dendrite formation at the lithium metal surface, protecting the lithium metal (and possibly other materials) from moisture, and avoiding the formation of reaction by-products due to electrochemically-induced electrolyte decomposition reactions.
This challenge of avoiding the formation of reaction by-products due to electrochemically-induced electrolyte decomposition is of particular importance because it has recently been demonstrated that during discharge of a Li-air battery cell containing propylene carbonate (PC) as electrolyte, Li2CO3 is the primary product rather than Li2O2. Mizuno, F., et al., Rechargeable Li-Air Batteries with Carbonate-Based Liquid Electrolytes; Electrochem. Soc. of Japan, 78(5):403-405 (2010). The origin of the carbonate-based electrolyte decomposition has been hypothesized to occur due to the presence of superoxide ion O2− species in the electrochemical environment of the Li-air battery systems Bryantsev, V. S., et al., Computational Study of the Mechanisms of Superoxide-Induced Decomposition of Organic Carbonate-Based Electrolytes.; The Jour. of Phys. Chem. Letters, 2: 379-383 (2011).
This species is a powerful reducing agent, which is thought to react very rapidly with a variety of organic substrates. See, e.g., Sawyer, D. T. et al., How Super is Superoxide?; Accts. of Chem. Res., 14(12): 393-400 (1981). Overall, these results point toward the irreversibility of carbonate-based electrolytes for Li-air cell batteries. As such, a battery having a solvent/electrolyte that resists breakdown by a superoxide ion would be greatly appreciated in the art.